1. Field of the Invention
The present invention relates to an optical modulator and an optical modulation method which are used in optical communications. For example, the present invention relates to an optical modulator and an optical modulation method which modulates light intensity on the basis of interaction between light and surface plasmons.
2. Related Art of the Invention
Optical modulation methods for optical communications are classified into a direction modulation scheme in which a light source is modulated using a driving current and an indirect modulation scheme in which propagation light is modulated by allowing the propagation light to interact with electrons and polarization in a transmission path controlled using an external physical quantity.
The direct modulation scheme is limited in terms of high-speed modulation owing to the presence of the threshold current and capacity of a light source. The indirect modulation scheme is thus used for high-speed modulation.
Typical examples of the indirect modulation scheme include a transmission type in which an electrooptical material such as LiNbO3 or KTP is utilized to substitute phase-modulated orthogonal polarization into intensity modulation through interference and a mode coupling type in which the resonance between surface plasmons (SP) and photons are utilized to modulate transmission light quantity.
However, with current materials, the transmission type needs to form such a waveguide as reduces electrode interval down to a 10-μm width in order to increase sensitivity up to a practical level. Realization of such a small electrode interval requires high manufacturing costs, resulting in high prices. Thus, the mode coupling type is advantageous in terms of costs.
One mode coupling type indirect modulation scheme applies a high-frequency voltage to the interface between metal and an insulator to resonate the insulator to generate surface plasmons localized at the interface between the metal and the insulator. The surface plasmons are coupled with propagation light in a waveguide provided adjacent to the insulator to modulate the propagation light (see, for example, Japanese Patent Laid-Open No. 5-313108).
FIG. 13 shows a side sectional view of a surface plasmon wave modulator disclosed in Japanese Patent Laid-Open No. 5-313108.
The surface plasmon wave modulator 110 has a flat multilayer structure made of two metal electrodes 118A, 118B, a photoelectric material 120 sandwiched between the metal electrodes 118A, 118B, two layers of a coating material 114 located opposite the photoelectric layer 120, and a flat single mode waveguide 112 sandwiched between the two layers of the coating material 114. Furthermore, a thin buffer layer 116 (0.1 μm) is provided between the metal electrode 118A and the coating material 114.
The waveguide 112 is formed of a transparent material such as glass. The waveguide 112 has an extremely small thickness of 6 μm and only one light wave mode occurs inside the waveguide 112.
The photoelectric material 120 is formed of a photoelectric polymer having a smaller refractive index than the waveguide 112. The refractive index of the photoelectric material 120 is varied by a voltage applied to the metal electrodes 118A, 118B.
Two different modes occur in the surface plasmon wave modulator 110, a core mode induced by the waveguide 112 and a surface plasmon wave interface mode occurring at the interface between the layers of the metal electrode 118A and the photoelectric material 120. Energy from the guided core mode is resonantly coupled to the surface plasmon wave interface mode when the phase speeds of the two modes match. The phase match conditions are controlled by applying an electric field to vary the refractive index of the photoelectric material 120.
A TM polarization component of a light wave is coupled to the surface plasmon wave interface mode. Consequently, the power of TM polarized incident light is coupled to the surface plasmon wave interface mode under the phase match conditions. Therefore, the output power level is controlled by the electric field applied to the photoelectric material 120.
However, with the conventional surface plasmon wave modulator, to allow surface plasmons localized at the interface between the insulator film (photoelectric material 120) and the metal film to resonate with modulated light, it is necessary to select, as an insulator film, a low-refractive-index material having a smaller refractive index than the waveguide 112 through which the modulated light propagates. Thus, the range of material selection is conventionally narrow. Disadvantageously, very few types of insulator materials have a smaller refractive index than glass and resin, which are commonly used as a waveguide material (glass and resin have a refractive index of about 1.5). Further, it has been impossible to use a high dielectric having a large electrooptical constant.
This problem will be described below. Specifically, the mechanism of optical modulation based on the resonance between surface plasmons and the waveguide mode will be described using dispersion properties.
First, description will be given of a case in which light is incident on a dielectric layer in a two-layer structure including the dielectric layer and a metal layer.
FIG. 14(A) shows a schematic sectional view of a case in which light is incident on a two-layer structure including a dielectric layer and a metal layer. FIG. 14(B) shows the properties of the dispersion of photons and surface plasmons in the dielectric layer in the two-layer structure including the dielectric layer and metal layer.
The inclination of a dispersion curve 132 for photons advancing through the dielectric layer 130 corresponds to a straight line determined by the refractive index of the dielectric layer 130 as shown in FIG. 14(B). In contrast, a dispersion curve 133 for surface plasmons localized at the interface between the dielectric layer 130 and the metal layer 131 always has a greater wave number than the dispersion curve 132 for the photons. The dispersion curve 133 draws a curve approximating a curve for a plasma frequency and has no intersecting point with the dispersion curve 132 for the photons in the dielectric layer. That is, the photons in the dielectric cannot in principle be coupled to the surface plasmons localized at the metal interface, located adjacent to the dielectric.
Now, description will be given of a case in which light is incident on a dielectric layer in a three-layer structure including a low-refractive-index dielectric layer, a metal layer, and a high-refractive-index dielectric layer.
FIG. 15(A) shows a schematic sectional view of a case in which light is incident on a three-layer structure including a low-refractive-index dielectric layer, a metal layer, and a high-refractive-index dielectric layer. FIG. 15(B) shows the properties of the dispersion of photons and surface plasmons in the dielectric layer in the three-layer structure including the low-refractive-index dielectric layer, the metal layer, and the high-refractive-index dielectric layer.
As shown in FIG. 15(B), when light is incident from the high-refractive-index dielectric layer 135, a dispersion curve 141 for photons advancing through the high-refractive-index dielectric layer 135 has no intersecting point with a dispersion curve 142 for surface plasmons 138 localized at the interface between the high-refractive-index dielectric layer 135 and the metal layer 136. When it is assumed that light is incident from the low-refractive-index dielectric layer 134, a dispersion curve 139 for photons advancing through the low-refractive-index dielectric layer 134 has no intersecting point with a dispersion curve 140 for surface plasmons 137 localized at the interface between the low-refractive-index dielectric layer 134 and the metal layer 136. Thus, as is the case with the two-layer structure, the photons in the dielectric cannot be coupled to the surface plasmons localized at the metal interface, located adjacent to the dielectric.
On the other hand, the dispersion curve 139 for the photons in the low-refractive-index dielectric always has a higher frequency than the dispersion curve 141 for the photons in the high-refractive-index dielectric. The dispersion curve 140 for the surface plasmons localized at the interface between the low-refractive-index dielectric layer and the metal layer always has a higher frequency than the dispersion curve 142 for the surface plasmons localized at the interface between the high-refractive-index dielectric layer and the metal layer. Thus, as shown in FIG. 15(B), the dispersion curve 141 for the photons in the high-refractive-index dielectric always has an intersecting point with the dispersion curve 140 for the surface plasmons localized at the interface between the low-refractive-index dielectric layer and the metal layer. That is, for the three-layer structure, the photons in the high-refractive-index dielectric can be coupled to the surface plasmons localized at the interface between the low-refractive-index dielectric layer and the metal layer.
The waveguide 112, metal electrode 118A, and photoelectric material 120 in the surface plasmon wave modulator 110, shown in FIG. 13, correspond to the high-refractive-index dielectric layer 135, metal layer 136, and low-refractive-index dielectric layer 134 in the three-layer structure, shown in FIG. 15(A). In the surface plasmon wave modulator 110, since the photoelectric material 120 has a smaller refractive index than the waveguide 112, the photons in the waveguide 112 can be coupled to the surface plasmons localized at the interface between the photoelectric material 120 and the metal electrode 118A.
However, in the surface plasmon wave modulator 110, if the photoelectric material 120 has a greater refractive index than the waveguide 112, a dispersion curve for photons in the waveguide 112 corresponds to the dispersion curve 139 for the photons in the low-refractive-index dielectric, shown in FIG. 15(B). A dispersion curve for surface plasmons localized at the interface between the metal electrode 118A and the buffer layer 116 and a dispersion curve for surface plasmons localized at the interface between the metal electrode 118A and the photoelectric material 120 correspond respectively to the dispersion curve 140 for the surface plasmons localized at the interface between the low-refractive-index dielectric layer and the metal layer and the dispersion curve 142 for the surface plasmons localized at the interface between the high-refractive-index dielectric layer and the metal layer, shown in FIG. 15(B). These dispersion curves have no intersecting point. That is, in this case, light advancing through the waveguide 112 cannot be coupled to the surface plasmons on any surface of the metal electrode 118A.
Therefore, the configuration of the surface plasmon wave modulator 110 in Japanese Patent Laid-Open No. 5-313108 needs to use a material having a smaller refractive index than the waveguide 112 as the photoelectric material 120.
On the other hand, a material having a greater electrooptical constant enables a reduction in voltage applied in order to vary the refractive index, facilitating control. This also enables a reduction in the size of a driving power source, allowing miniaturization and an increase in modulation speed.
However, materials having greater electrooptical constants generally have greater refractive indices than glass and resin. Thus, with the configuration of the surface plasmon wave modulator 110 in Japanese Patent Laid-Open No. 5-313108, materials having greater electrooptical constants have not been able to be used as a dielectric layer (photoelectric layer 120).